The present invention relates to a method and apparatus for performing temporally resolved wavelength measurements and power measurements, of particular application in an optical spectrum analyzer in which measurements performed in the time domain are used to calculate the wavelength and power spectrum of a laser or other light source.
In applications such as optical telecommunications there is a need for small, portable, rugged and low cost devices that can accurately measure the wavelength and power spectrum of a laser or other light source with improved resolution, increased measurement speed and reduced cost. Parameters of importance in optical spectrum analyzers include wavelength range, wavelength resolution, wavelength accuracy, optical sensitivity, power calibration accuracy and dynamic range.
Existing devices, such as commercially available optical spectrum analyzers, employ spatially resolved methods in which measurements of spatial parameters are used to calculate wavelength spectra of the input light. Generally, input light is firstly collimated and then spectrally dispersed by a diffraction grating. The resulting diffracted light is then typically reflected from several mirrors or diffraction gratings before exiting a slit aperture and is detected by a photodetector. The wavelength range is scanned by adjusting the angle of the diffraction gratings, the position of the photodetector, or both. The spatially resolved data is translated to wavelength data by calibrating against a known reference source. The result is a wavelength spectrum relating the relative intensity of the light source at a measured position or angle to the wavelength of the light source over a given range of values. A feature common to existing systems, therefore, is the performance of measurements in the spatial domain and the collection of data while varying a parameter such as detector position or grating angle. The relative accuracy and resolution of this technique is dependent on the relative positioning accuracy and translational stability of both moving and fixed mechanical components.
U.S. Pat. Nos. 5,233,405 and 5,359,409 disclose similar double-pass scanning monochromator designs for use in an optical spectrum analyzer device, in which an input light beam is spatially dispersed by a diffraction grating and passed through a slit so that a portion of the dispersed light beam can be selected. The monochromator based optical spectrum analyzer includes a motor for rotating the diffraction grating and a shaft angle encoder for sensing grating position. The light that passes through the slit is recombined to produce an output light beam. The input light beam is incident on the diffraction grating during first and second passes. A polarization rotation device rotates the polarization components of the light beam by 90xc2x0 between the first and second passes so that the output of the monochromator is independent of the polarization of the input light beam. An output optical fiber is translated by a micropositioning assembly in a plane perpendicular to the output light beam during rotation of the diffraction grating to automatically track the output light beam and to provide optical chopping.
U.S. Pat. No. 5,497,231 discloses a monochromator design utilizing a beam-diffracting scanning mirror on a oscillated spring. The spring acts as an electromechanical self-energized oscillation circuit; a sensor detects the deflection of the spring, its output used as a feedback signal for maintaining the spring""s oscillation.
U.S. Pat. No. 5,886,785 discloses an optical spectrum analyzer for an incident light beam and a process for analyzing the corresponding spectrum. The spectrum analyzer comprises addressing means, a diffraction grating, a reflecting dihedron, a device for adjusting the rotation of the reflecting dihedron and reception means. A polarization separator divides the incident beam into first and second parallel secondary beams of linearly polarized light along the directions parallel to and perpendicular to the grooves in the grating respectively, and a xcex/2 plate placed on the path of the first secondary beam applies a perpendicular polarization direction to this beam. The grating diffracts the secondary beams a first time, the reflecting dihedron exchanges their directions, the grating diffracts them a second time, the xcex/2 plate applies a 90xc2x0 rotation to the polarization state of the second secondary beam and the separator recombines the secondary beams into a single main beam returned to reception means.
U.S. Pat. No. 6,097,487 discloses a device for measuring wavelength, including an interrogation broadband light source and a tunable optical filter. A first portion of the light transmitted through the filter and reflected from, or transmitted through, a fiber Bragg grating of known Bragg wavelength to provide an absolute wavelength reference, and directed to a first detector. A second portion of the light is transmitted through the filter and transmitted through, or reflected from a Fabry-Perot filter with fixed and known free spectral range to create a comb spectrum sampling the interrogation source spectrum to provide an accurate frequency/wavelength scale.
However, there are physical limits to parameters such as grating spacing and slit aperture size. Improvements in resolution and wavelength range require either increased optical path lengths or additional scanning elements (such as secondary diffraction gratings). A greater number of moving parts increases the complexity, instability and cost of the apparatus. Design instability and susceptibility to shock can produce inaccurate measurements in environments that require portability and ruggedness. Consequently improvements in wavelength resolution, scanning speed and scanning range are generally achieved at the expense of increased cost or size, and reduced portability.
Existing optical spectrum analyzers relate the wavelength of light to data measured in the spatial domain by varying and monitoring a parameter such as detector position or grating angle. The relative accuracy and resolution of this technique depends on the relative positioning accuracy and translational stability of moving and fixed mechanical components. The construction of accurate and repeatable mechanical translation and oscillation assemblies leads therefore to a high cost of manufacture.
U.S. Pat. No. 4,732,476 discloses a rapid scan spectrophotometer device that measures the spectral transmission of sample materials that are illuminated by a broadband light source. The spectrophotometer device is a different type of apparatus from an optical spectrum analyzer and has different functions and applications. The disclosed spectrophotometerxe2x80x94in order to measure the relative wavelength parameterxe2x80x94relates that parameter to the temporal difference between detected signals of different wavelengths as they are scanned past a photodetector at a constant speed. Hence, the relative transmission spectra is calculated using, in part, a temporally resolved measurement technique. However, although this spectrophotometer design has advantages in device cost and complexity, its calibration accuracy depends on the incorporation of spatial measurements and the positioning stability of mechanical components.
It is an object of this invention to provide a new temporally resolved method of measuring at least some properties of a light source.
The present invention provides, therefore, an apparatus for determining the wavelength of a sample source of light, said apparatus having:
a reference light source of known wavelength;
a collimator for collimating light from said sample source and from said reference source;
a dispersing means for receiving and spatially dispersing collimated light from said collimator according to wavelength;
focusing means for focusing dispersed light from said dispersing means; and
a photodetector located in the focal plane of said focusing means and having an aperture for spatially selectively admitting light from said focusing means, and operable to provide a temporally calibratable output signal indicative of the wavelength of said selectively admitted light;
wherein said apparatus is operable to scan said focused spatially dispersed beam across said aperture, and said photodetector output includes resolvable features corresponding to light from said reference source and sample source, whereby a time difference between said features is indicative of a wavelength difference between said light from said reference source and said sample source.
Preferably said apparatus includes a beam splitter and a further photodetector, said beam splitter directing some of said light from said sample source and from said reference source to said further photodetector, wherein said further photodetector is operable to provide an output signal indicative of said power of said sample source.
Preferably said light from said reference source is coupled to said light from said sample source optically after said beam splitter and optically before said collimator, to provide collinear beam propagation without affecting the accuracy of power measurements of the sample light.
Preferably said apparatus is operable to determine the integrated power of said sample source independent of the wavelength spectrum of said sample source, and said further photodetector output is indicative of the total power of said light from said sample source.
The photodetectors may be provided as a single photodetector.
Preferably said apparatus includes an input aperture for admitting said light from said sample source into said apparatus.
Preferably said beam splitter comprises a partially reflecting mirror.
Preferably said apparatus is operable to avoid or minimize unwanted detection of light from said reference source by removing signals indicative of said unwanted light from sample data by gating or time multiplexing said light from said reference source and said light from said sample source.
Preferably said apparatus directs said focused, dispersed beam in a direction that rotates relative to said aperture with substantially constant angular velocity.
Thus, the focused light is scanned across the aperture so that output spectrum can be produced that is effectively sample source intensity as a function of time (relative to the reference source) or, as the wavelength of the reference source is known, versus wavelength. Light may be admitted from the sample light source from the tip of an optical fiber or the like; the light itself may be from, for example, a laser.
The dispersing means may be either a mechanical dispersing means, such as a diffraction grating or prism, or a non-mechanical dispersing means, such as those employing acousto-optic, electro-optic or non-linear beam dispersing methods, or combinations of both mechanical and non-mechanical dispersing means.
It should be understood that the aperture may take many forms, including a physical aperture in a mask over the photodetector, the core of an optical fiber (the core providing the spatial filtering) or simply that portion of the face of the photodetector that admits light for detection.
In embodiments in which the dispersing means is mechanical, the relative rotation of the spatially dispersed beam will generally be provided by rotating the dispersing means relative to the other optical elements. This will most commonly be achieved by rotating the dispersing means, but it is envisaged that, in some embodiments, the dispersing means will be stationary while some or all of the other optical elements are rotated as necessary to scan the spatially dispersed beam across the aperture. In the latter case, the dispersing means would, though stationary, be rotating in the reference frame of those other elements.
Preferably said dispersing means is a diffracting means, and more preferably a rotatable diffraction grating. In one embodiment, the dispersing means is a rotatable prism.
Preferably said reference source is a stable light source of ultra-narrow linewidth.
This narrow linewidth reference source could be provided, however, in the form of a broadband source with a suitable filter (such as an acetylene absorption cell filter).
Preferably said collimator is a collimating mirror. Preferably said focusing means is a focusing mirror.
However, any other suitable collimators could be employed, such as a collimating lens. Similarly, a focusing lens could be used as the focusing means.
Preferably said apparatus is operable to use said reference source to measure the speed of revolution of said dispersing means.
Preferably said apparatus includes a wavelength data analysis system for resolving said photodetector output signal with respect to time and translating the temporal data to calibrated wavelength data.
Preferably said apparatus is operable as a single pass scanning wavelength measurement device. Alternatively, said apparatus is operable as a double pass or multiple pass scanning wavelength measurement device.
Preferably the apparatus includes means for time averaging and statistically analysing collected data so that more accurate power and wavelength data can be produced from said apparatus.
Preferably the apparatus includes a plurality of photodetectors disposed about said dispersing means, to improve resolution, accuracy, dynamic range or sampling rate.
Thus, an advantage of this embodiment is that, as the spectrum is constantly moving relative to the photodetector, it has the chance to pass via several detectors that can measure several different parameters in several different ranges. The speed of the rotation can be adjusted to ensure a large enough signal. Also, the size of the detector apertures and detector response capability all relates to the practical resolution, accuracy and dynamic range achievable with the apparatus.
Preferably the apparatus includes a plurality of dispersing means to increase resolution, accuracy or sampling rate.
Preferably said apparatus is operable to measures the relative power versus wavelength and total power spectrum, includes a plurality of photodetectors, and calibration means for calibrating the amplitude of the outputs of said photodetectors versus wavelength.
The present invention further provides a method of determining wavelength of a sample source of light, involving:
providing a reference beam of light of known wavelength;
collimating light from said sample source and collimating light from said reference beam;
dispersing said collimated light to produce a spatially dispersed beam;
focusing said dispersed beam;
scanning said focused, dispersed beam across a detector aperture and to thereby spatially selectively detect focused light and produce a temporally calibratable output signal indicative of said selectively detected light; and
resolving features corresponding to said light from said sample source and to said reference beam;
whereby a time difference between said features is indicative of a wavelength difference between said reference beam and said light from said sample source.
Preferably the method includes:
beam splitting said light from said sample source and said reference beam;
directing some of said light from said sample source and some of said reference beam to a further detector; and
generating an output signal indicative of said power of said sample source.
Preferably the method includes coupling said reference beam to said light from said sample source after said beam splitting and before said collimating, to provide collinear beam propagation without affecting the accuracy of power measurements of the sample light.
Preferably the method includes determining the integrated power of said sample source independent of the wavelength spectrum of said sample source, wherein said output signal is indicative of the total power of said light from said sample source.
Preferably said detector and said further photodetector are provided as a single photodetector.
Preferably the method includes admitting said light from said sample source through an input aperture.
The collimated beam may be dispersed by means of any suitable mechanical or non-mechanical dispersing means. Preferably the collimated beam is dispersed by means of a diffracting means, and more preferably by means of a rotatable diffraction grating. Preferably said method includes using said reference beam to measure the speed of revolution of said diffracting means.
Preferably the method includes detecting said focused light by means of a plurality of photodetectors disposed about said dispersing means, to improve resolution, accuracy or sampling rate.
When a plurality of photodetectors is employed in an optical spectrum analyzer, the amplitude of the outputs from the photodetectors is preferably calibrated versus wavelength. In one example of such an arrangement, the plurality of photodetectors includes a reference measurement of said reference beam in the form of a thermoelectric detector whose measurement is independent of wavelength.
Preferably said method includes directing some of said reference beam and some of said light from said sample source to a photodetector and determining the power of said sample source. This may be done by means of a beam splitter, preferably in the form of a partially reflecting mirror. There are numerous possible embodiments of the beam splitter including the use of 4% Fresnel reflection from an uncoated piece of glass or other beam sampling means. The sampled portion of the input beam can be used as the reference measurement of optical power. All measurements of optical intensity by the plurality of calibrated detectors can then be referenced back to this wavelength independent reference power measurement.
Preferably the method includes increasing any one or more of resolution, accuracy and sampling rate by employing a plurality of diffracting means.
Preferably the method includes providing said reference beam by means of a stable light source of ultra-narrow linewidth.
In one embodiment, said reference beam is provided by means of a broadband light source with a suitable filter (such as an acetylene absorption cell filter).
Preferably said light from said sample source and said reference beam is collimated by means of a collimating mirror. Preferably said dispersed beam is focused by means of a focusing mirror.
Preferably said method includes translating temporal data to calibrated wavelength data.
Preferably said method is a single pass scanning method. Alternatively, said method is a double pass or multiple pass scanning method.
Preferably the method includes time averaging and statistically analysing collected data to produce more accurate wavelength data.
Preferably the method includes converting acquired temporal data into spectral and power measurements.
Preferably the method includes calculating the relative power distribution by calibrating the intensity of output signal, from a photodetector with the known wavelength response of the photodetector material and the total power of the sample source. Preferably said photodetector is a fast response photodetector.
Preferably the method includes measuring the total power distribution by performing time integrated measurements on a fixed portion of the sample beam, preferably by using a wavelength independent slow response photodetector such as a thermopile or pyroelectric detector.
In yet another embodiment, the system can be operated without a reference light source present during measurement of said light from said sample beam. In one embodiment, a mirror reflection is obtained from a rotating mounting of a dispersing means. This may be a mirror on the rear of the dispersing element or more preferably in the case of a rotating diffracting means may be the zero order diffracted beam which includes all wavelengths present in said sample beam.
This zero order diffracted beam may be used to provide the timing marker for measurement of angular velocity. An initial calibration of the system may be performed using a reference optical source whereby rotation rate is linked to temporal spread of the known optical reference source. Said calibration information of spectral spread versus rotational speed can then be stored in the device.
In yet another embodiment, said beam splitter is a fiber-optic beam splitter that transmits the majority of the input signal for wavelength measurement via an optical fiber output. This output fiber may be connected to the output fiber from said reference source which is coupled to the input sample source via a coupling device that is positioned after the beam splitter.
This arrangement provides good collinear propagation of the reference and sample beam propagation into the wavelength measurement portion of the apparatus. It also means that the reference and sample sources could be operated simultaneously and this design would not require gating or time multiplexing of the reference and sample sources to avoid potential errors in total power measurements.